A typical EGR system of an engine includes one or more EGR valves for controlling the flow of engine exhaust gas from the engine's exhaust system to the engine's intake system to meter an appropriate amount of exhaust gas into fresh air passing through the intake system where the air supports combustion of fuel in the engine's cylinders. The metered exhaust gas in effect dilutes the air so that in-cylinder temperature rise resulting from combustion is limited from that which would occur in the absence of such dilution. As a consequence, the quantity of oxides of nitrogen (NOx) in the exhaust gas that results from combustion is also limited.
Some EGR systems, especially those designed for compression ignition (i.e. diesel) engines, have one or more heat exchangers for cooling recirculated exhaust gas. Cooling of the exhaust gas can further limit the generation of NOx.
It is recognized in the industry that cooling of recirculated exhaust gas creates the potential for condensation of certain gaseous constituents of the exhaust gas. Control of condensation may be a factor in the design of various engine systems.